The proliferation of digital communications systems continues to drive the needs for low cost, high performance radio receivers. This has led to the development of direct conversion and complex digital intermediate frequency (IF) receivers. Complex digital IF receivers generally include two stages. The first stage converts a received signal to a complex digital signal at an intermediate frequency without the analog filters typically associated with a double down conversion receiver. The second stage converts the complex digital IF signal to a digital baseband data signal. Both direct conversion and complex IF double conversion receivers implement a form of quadrature modulation, typically processing a received radio frequency (RF) signal along in-phase (I) and quadrature (Q) pathways.
In an ideal situation, the signals in the I and Q paths would have identical levels of gain and a phase offset of precisely 90°. However, in practice these paths are not ideal. The electrical characteristics of each path can vary with respect to the other, typically due to deviations in the fabrication process. The gain, phase and direct current (DC) offset of the I and Q signals propagating through each path are individually affected by the specific electrical characteristics of each path, as well as variations in operating conditions and drift in the frequency of the received RF signal. Any difference in the gain or phase between the in-phase and quadrature signals is undesirable error, which can prevent the information carried in those signals from being properly retrieved.
Typical quadrature communication systems deal with the effects of corruption through the design of a receiver architecture that avoids these effects, such as the traditional double down conversion receiver or through the design of direct conversion and complex IF double conversion receiver architectures that adhere to strict tolerances over a time and temperature range. The goal of these latter designs is to implement I and Q paths that are identical in gain and maintain a phase offset of 90° independent of the frequency of the received signal. An attempt to obtain this matched condition over time and temperature can require the addition of complex analog circuitry and the use of specialized fabrication processes, both of which add significant time and cost to the development and production of communication systems and still may not achieve the desired performance.
Other conventional systems have attempted to alleviate gain error through non-coherent calibration techniques, where a calibration signal is propagated along the I and Q paths of the receiver. The gain error is measured by squaring these I and Q signals. For example, the squaring of the in-phase signal generates a DC component having an amplitude proportional to the amplitude of the in-phase calibration signal according to well known mathematical principles. However, such non-coherent detection also results in the generation of a series of unwanted DC components due to noise and interference present in the signals. The DC components hamper any measurements made during the calibration process and result in inaccurate measurements of the amplitudes of the I and Q signals.